Median eye in larvae of Hydryphantes ruber (De Geer, 1778) (Acariformes: Hydryphantidae)

Abstract

Median eye in larva of the water mite Hydryphantes ruber (De Geer, 1778) was investigated by means of light-optical and electron-microscopical (SEM and TEM) methods. The median eye is a single organ situated at the dorsal body surface under a slightly thickened and protruded cuticle in the middle of the dorsal shield. The median eye is a simply organized structure and is composed of small retinula cells with everted mostly isolated rhabdomeres looking to the cuticle. Each rhabdomere consists of irregularly arranged microvilli. The basal nuclear zones of these cells are occupied by electron-dense pigment granules. Two laterally situated nerves are composed of five axons each that suggest the paired origin of the median eye. Epidermal cells reach in lipid inclusions totally enclose the eye and are underlain by a flat basal lamina. Retinula cells contact to each other and to the epidermal cells by septate junctions. The larval median eye appears to be rather similar to median eye in adult mites of this species studied previously (Mischke 1981).